EXSS Nutrition Ch 3: Fuel Sources for Muscle and Exercise Metabolism

Skeletal muscle cells

Long, striated, multinucleated fibers commonly called myofibers.

Myofibrils

The contractile elements of muscle fibers, composed of sarcomeres.

Sarcomeres

The functional contractile units containing thin actin and thick myosin filaments.

Actin

Thin filaments in the sarcomere.

Myosin

Thick filaments in the sarcomere; myosin heads form cross-bridges with actin.

Cross-bridges

Myosin heads that bind reversibly to actin and cause the filaments to slide toward the center of the sarcomere.

Type I fibers

Slow-twitch fibers with high oxidative capacity and extreme fatigue resistance.

Type IIa fibers

Fast-twitch, fatigue-resistant fibers with characteristics between Type I and Type IIX.

Type IIX fibers

Fast-twitch, highly fatigable fibers specialized for rapid, powerful contractions.

Type I fiber characteristics

High oxidative capacity, many mitochondria, slow myosin ATPases, and high fatigue resistance.

Type IIX fiber characteristics

Fast myosin ATPases, fewer mitochondria, poorer capillary supply, greater glycogen/phosphocreatine stores, and rapid fatigue.

Type IIa fiber characteristics

Fast-acting myosin ATPases with oxidative capacity more similar to Type I fibers.

Muscle fiber recruitment

An orderly progression that generally goes from Type I → Type IIa → Type IIX.

Light exercise

Primarily uses Type I fibers.

Moderate exercise

Uses Type I and Type IIa fibers.

Severe/high-intensity exercise

Uses all three fiber types.

Muscle fiber plasticity

Muscle fibers can adapt to training, changing their metabolic potential even though fiber-type distribution is largely genetically determined.

Endurance athletes

Typically have a high proportion of Type I fibers.

ATP

Adenosine triphosphate; the only energy source that can be used directly for muscle contraction and other energy-requiring cellular processes.

ATP's role in muscle

ATP hydrolysis by myosin ATPase provides energy that causes muscle fibers to shorten.

ATP

Acts as the body's primary energy molecule or "molecular unit of currency."

ATP concentration

ATP must be continually resynthesized because depletion would be fatal to the cell.

Metabolism

The use of energy for bodily processes, including all chemical changes.

Anabolism

Metabolic processes that use energy to build molecules and tissues.

Catabolism

The breakdown of molecules to generate usable energy or create building blocks for anabolism.

Metabolic pathway

A series of chemical reactions that can produce catabolic or anabolic outcomes.

Glycolysis

A metabolic pathway that breaks down glucose and occurs in the cytosol.

ATP resynthesis

The process of producing ATP again from ADP so the cell can continue performing work.

Three major ways ATP is resynthesized

Phosphocreatine hydrolysis, anaerobic glycolysis, and aerobic oxidation.

Phosphocreatine hydrolysis

The breakdown of phosphocreatine to provide phosphate for rapid ATP resynthesis.

Anaerobic metabolism

ATP production from glycogen or glucose through glycolysis without adequate oxygen.

Aerobic metabolism

ATP production through oxidation of acetyl-CoA, primarily from carbohydrate and fat, in the presence of oxygen.

Law of energy conservation

Energy cannot be created or destroyed; it is transformed from one form to another.

Chemical energy

The energy derived from molecular bonds in carbohydrate, fat, and protein.

Energy from food

Chemical energy from food is transferred into cellular energy used to perform physiological tasks.

Three stages of extracting energy from food

Digestion/absorption/transport; breakdown into metabolites; and use of metabolites to produce usable energy.

Stage 1 of energy extraction

Energy-yielding nutrients are digested, absorbed, and transported.

Energy-yielding nutrients

Simple sugars from carbohydrate, amino acids from protein, fatty acids from lipids, and alcohol.

Stage 2 of energy extraction

Food-derived molecules are further broken down into metabolites.

Pyruvate

A three-carbon metabolite produced when glucose is broken down during glycolysis.

Stage 3 of energy extraction

Cells use energy-producing metabolites to produce a usable form of energy.

Energy stores

Stored energy in forms such as glycogen and fat that can be used to continuously synthesize ATP.

Cytoplasm

The semifluid area inside the cell membrane that contains organelles and is the site of glycolysis.

Cytosol

The fluid portion of the cytoplasm where glycolysis occurs.

Mitochondria

The "powerhouse of the cell"; the major site of aerobic energy production.

Mitochondrial matrix

The inner region of the mitochondria where the TCA cycle occurs.

Mitochondrial membranes

Mitochondria have an outer membrane and a highly folded inner membrane surrounding the matrix.

TCA cycle

Also called the Krebs cycle or citric acid cycle; a series of reactions in the mitochondrial matrix that helps produce ATP.

Krebs cycle

Another name for the TCA cycle.

Citric acid cycle

Another name for the TCA cycle.

Acetyl-CoA

A molecule formed from carbohydrate, fat, and protein metabolism that enters the TCA cycle.

Oxidative decarboxylation

A process that breaks carbon bonds and produces electrons carried by coenzymes to the electron transport chain.

Electron transport chain (ETC)

A series of protein complexes that accept electrons from coenzymes and use their energy to help produce ATP.

Oxidative phosphorylation

The final step of ATP formation, using energy from electrons in the ETC.

NAD+

A coenzyme involved in carrying electrons from fuel breakdown to the electron transport chain.

FADH2

A coenzyme that carries electrons from fuel breakdown to the electron transport chain.

ATP structure

ATP contains three phosphate groups attached to adenosine.

ATP hydrolysis

Breaking phosphate bonds in ATP releases energy that can power biological work.

ADP

Adenosine diphosphate; produced when ATP loses a phosphate group.

AMP

Adenosine monophosphate; produced when ADP loses another phosphate group.

ATP as energy storage

ATP is not considered an energy-storage molecule; its production rate changes with muscle mass and energy demands.

Substrate

A molecule acted upon by an enzyme to create different metabolic products.

Four major fuel substrates

Carbohydrate, fat, protein, and phosphocreatine.

Phosphocreatine

A high-energy compound in muscle that provides phosphate for rapid ATP resynthesis.

Phosphagen system

An energy system that uses phosphocreatine exclusively to regenerate ATP in muscle tissue.

Three energy systems

Phosphagen, anaerobic, and aerobic.

Energy system characteristics

Complexity of pathways, rate of ATP production, capacity to produce ATP, and lag time.

Rate of ATP production

How quickly an energy system can produce ATP.

Capacity of ATP production

How much ATP an energy system can produce.

Lag time

The time required for an energy system to contribute significant amounts of ATP when demand increases.

Phosphagen energy system

Uses phosphocreatine exclusively and produces ATP virtually instantaneously.

Phosphagen ATP production rate

The fastest of the three energy systems.

Phosphagen activity duration

Primarily supports activities lasting about 10 seconds or less.

Phosphagen examples

Heavy weightlifting and sprinting.

Anaerobic energy system

Uses glycolysis and produces ATP rapidly, especially during high-intensity exercise after the phosphagen system is depleted.

Glycolysis

A 12-step process that breaks down glucose and produces ATP and pyruvate.

Anaerobic ATP production rate

The second-fastest ATP production rate, close behind the phosphagen system.

Anaerobic activity duration

Supports all-out exercise efforts beyond about 10 seconds.

Anaerobic exercise examples

Heavy weight training and interval training.

Anaerobic glycolysis

Glycolysis occurring without adequate oxygen delivery to the mitochondria.

Lactate

In anaerobic glycolysis, much of the pyruvate is converted to lactate.

Aerobic energy system

The most complex energy system; produces ATP using oxygen and can use any macronutrient.

Aerobic ATP production rate

Slower than the phosphagen and anaerobic systems.

Aerobic ATP production lag

Can take minutes to produce enough ATP for a physical workload because oxygen must be delivered to the mitochondria.

Aerobic capacity

The maximal amount of oxygen a person can use in one minute per kilogram of body weight.

Aerobic ATP capacity

Can be considered unlimited when adequate oxygen is available.

Aerobic system at rest

The body's primary ATP production system when at rest.

Aerobic exercise examples

Long-distance running, swimming, rowing, and cycling.

Aerobic pathways

Beta-oxidation, glycolysis, deamination, TCA cycle, and electron transport chain.

100-yard sprint

Primarily uses the phosphagen energy system.

Interval training

Primarily uses the anaerobic energy system.

Marathon swimming

Primarily uses the aerobic energy system.

30-minute walk

Primarily aerobic, with anaerobic and phosphagen systems contributing as secondary/third systems.

Soccer game

Uses both anaerobic and aerobic energy systems, with the phosphagen system also contributing.

Marathon

Primarily aerobic, with anaerobic and phosphagen systems contributing.

Anaerobic glycolysis ATP yield

One molecule of glucose produces two molecules of pyruvate and a net two ATP.

Carbohydrate metabolism

Carbohydrate can provide energy anaerobically with lactate as an end product or aerobically through complete oxidation to carbon dioxide and water.

Carbohydrate depletion

Muscle glycogen can be rapidly depleted during exercise, while liver glycogen can be depleted during fasting.

Muscle glycogen depletion

Muscle glycogen stores are normally depleted after about 1–2 hours of hard exercise.

High-intensity exercise and glycogen

Carbohydrate is the major fuel for high-intensity exercise; when muscle glycogen is depleted, only low-intensity exercise is possible.